The goals of this project are to gain an understanding of rickettsia: endothelial cell interactions which result in: (i) rickettsial growth within vascular endothelia, and (ii) development of the vascular pathology associated with rickettsial diseases. Studies of rickettsial adherence and entry into human endothelial cells will utilize a novel radioactive assay which can distinguish between adherent and intracellular richettsiae. These studies, which will examine specificity, mode, kinetics and energy requirements for adherence and internalization should provide insight into the process by which an obligate intracellular parasite enters its primary target cell. Because rickettsioses are marked by extensive thrombogfenesis in the peripheral vaculature and endothelial cells produce substances which either participate in (Factor VIIIVWF) or regulate (PGI2) thrombogenesis, rickettsiae will be cultured in endothelial cells and infected endothelial cells will be assessed for changes in genral metabolic activity (phagocytic activity, 3H-leucine uptake), and production of clotting effectors (Factor VIIIVFW, PGI2). Furthermore, because rickettsiae have been shown to lyse several types of host cells (erythrocytes, fibroblasts, PMN) and high doses of rickettsiae are toxic to some animals, high doses of rickettsiae will be assessed for their immediate effect on endothelil cell integrity. endothelial cells incubated with high multiplicites of rickettsiae will be assessed for loss of cytoplasmic and membrane constitutents. Lysis of the vascular endothelium would result in exposure of the highly thrombogenic subendothelium and release of chemical effectors such as ADP, arachidonic acid an the prostaglandins E. Rickettsiaw will aslo be assessed for their ability to adhere to and penetrate into platelets and to initiate platelet aggregation directly or via endothelial cell damage. Studies on the immediate and delayed effects of rickettsiae on vascular endothelium and platelets should provide valuable insight into the pathogenesis of rickettsial disease.